Project Summary Chronic exposure to stressful experiences can result in maladaptive affective states that yield behavioral disturbances in rodents and stress-related mood disorders in humans. Over the last decade, the neural circuits underlying these maladaptive effects of stress have become better defined. One region of importance is the bed nucleus of the stria terminalis (BNST), which is a major output pathway connecting the central amygdala to the ventromedial nucleus of the hypothalamus that also receives direct projections from other limbic areas. Therefore, BNST may be an integrative center for limbic information and valence monitoring. Psychological and physiological stressors affect males and females differently. The BNST is a sexually dimorphic structure that may contribute to distinct chronic stress responses in males and females because expression of aromatase and both estrogen receptors (ER) ??? differs in male and female BNST. Direct activation of neurons within the oval nucleus of the BNST (ovBNST) increases anxiety-associated negative valence behaviors in male rodents, and our preliminary data demonstrates that exposure of C57BL/6J male mice to chronic variable mild stress (CVMS) results in increased corticotropin releasing hormone (CRH/CRF) signaling, increased mEPSC amplitude, altered resting membrane potential, and diminished M-currents in ovBNST neurons. While these data suggest that ovBNST may be a nexus for the effects of chronic stress on affective states, many questions remain unanswered. Our overall hypothesis is that CRH-expressing ovBNST neurons are critical mediators of the chronic stress response and that sexual dimorphism in the BNST underlies the distinct chronic stress responses found in males and females. The proposed specific aims will directly answer these questions and increase our understanding of how ovBNST mediates the maladaptive effects of chronic stress. In Aim 1, we will identify the cell populations that are impacted by chronic stressors (CVMS, chronic nondiscriminatory social defeat stress [CNSDS]) and the neurophysiological consequences in male and female mice. In Aim 2, we will assess how chronic stress affects ER? signaling in BNST and whether optogenetic modulation of ER?-positive BNST neurons mimics and/or reverses the effects of chronic stress on behavior. In Aim 3, we will determine the necessity and sufficiency of CRH-signaling in ovBNST neurons in mediating the behavioral effects of CVMS and CNSDS.